A Long Term Evolution, LTE, radio access network, RAN, uses a flat architecture with a single type of LTE radio base station, also denoted radio access node, eNB, eNodeB, or evolved NodeB. The radio base station is responsible for all radio related functions in one or several cells. LTE supports downlink transmission using a multi-antenna configuration. The multi-antenna configuration is used in LTE to support different transmission modes, TMs, for example transmit diversity and spatial multiplexing or multiple-input multiple-output, MIMO.
TM1 is a single antenna transmission mode, where only a single antenna is used.
TM2 is a transmit diversity mode. Operation according to TM2 amounts to sending the same information via different antennas. This improves the signal-to-noise ratio, SNR, and thus makes transmission more robust.
TM3 is also referred to as open loop spatial multiplexing, OLSM. This MIMO mode supports spatial multiplexing of up to two or up to four layers which are multiplexed onto two to four antennas, respectively. TM3 is used in order to achieve higher data rates. Due to the open loop property, TM3 does not require very much interaction with, i.e., feedback from, the user equipment, UE.
TM 4 is also referred to as closed loop spatial multiplexing, CLSM. Similar to TM3, this MIMO mode also supports spatial multiplexing of up to two or up to four layers which are multiplexed onto two to four antennas, respectively. This mode also achieves higher data rates compared to, e.g., TM2 or TM1. The closed loop property refers to that the UE continuously sends feedback regarding the channel situation, which, e.g., includes information about the preferred type of precoding.
In case of multi-antenna transmission in LTE downlink up to four cell specific logical antenna ports are defined, wherein each logical antenna port is mapped to one or more power amplifiers and physical antennas in the eNodeB. The LTE system considered in the present disclosure is configured with four logical antenna ports.
Traditionally, when one physical antenna fails, elements of a physical antenna branch fails or functions poorly, e.g. a situation with a bad antenna, failure of a power amplifier or broken filter, in a four-antenna eNodeB, the system either continues to work with the three remaining physical antennas while still maintaining a four antenna precoding scheme, i.e., continues as if nothing has happened. Alternatively, conventional LTE antenna fault handling methods are used, whereby a two antenna configuration replaces the previous four antenna configuration, since the LTE only allows one, two, or four antenna configurations.
Both of the above suggested approaches provide working solutions for handling a situation with one faulty antenna. However, both approaches result in significant draw-backs with regard to utilization of the system capacity and resources. For instance, in the case where transmission is continued over three physical antennas as if nothing has happened, a transmit diversity scheme will lose one diversity or main branch signal. For the cases where a two port configuration is selected, a fully functional physical antenna element is disconnected. Consequently, a failure of one physical antenna element results in loss of one additional physical antenna element, i.e., a sub-optimal use of transmit power capacity and a reduction in the ability for spatial diversity in the antenna system.